Flux composition and corresponding soldering method

ABSTRACT

A new flux composition, as well as corresponding methods for soldering electronic components to printed circuit boards, is disclosed. The new flux composition includes pimelic acid and two organic solvents. Significantly, the new flux composition leaves essentially no ionic residues at the completion of the soldering processes used to mount electronic components onto printed circuit boards.

This is a divisional of copending application, Ser. No. 08/251,806 filedon May 31, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a flux composition and to a correspondingmethod for soldering, for example, a semiconductor chip or a chipcarrier module to a printed circuit board.

2. Description of the Related Art

Fluxes play an important role in the procedures used to mount electroniccomponents onto printed circuit cards and printed circuit boards (bothof which are hereinafter generically referred to as printed circuitboards or PCBs). For example, one method for directly mounting asemiconductor integrated circuit device (hereinafter denominated asemiconductor chip or just a chip) onto a PCB is, for example, to formregions of solder, e.g., solder balls, on contact pads on thecircuit-bearing surface of the chip. Such solder regions may also beformed on corresponding contact pads on the PCB. Then, a flux is appliedto the solder regions on the chip and/or to the corresponding contactpads and/or corresponding solder regions on the PCB in order to removeoxide layers which may have formed on these solder regions or contactpads and to achieve increased wetting of the contact pads by the solderregions. Thereafter, with the circuit-bearing surface of the chip facingthe PCB, the solder regions on the chip are brought into contact withthe corresponding contact pads or solder regions on the PCB, and theresulting assembly is heated in order to melt, and thereby reflow, thesolder regions on the chip and/or on the PCB. Upon cooling andre-solidification, the resulting solder connections between the chip andthe PCB are typically encapsulated in an encapsulant, e.g., an epoxyresin encapsulant, to relieve any strains which may be engendered by adifference between the coefficient of thermal expansion (CTE) of the PCBand the CTE of the chip.

In a manner similar to that described above, one method for mounting amodule, e.g., an organic module or a ceramic module, bearingsemiconductor chips (hereinafter denominated a chip carrier module orjust module) onto a PCB, involves forming, e.g., screening, regions ofsolder onto contact pads on the non-chip-bearing surface of the module.Such solder regions may also be formed on corresponding contact pads onthe PCB. A flux is then applied to the solder regions on the moduleand/or to the corresponding contact pads and/or corresponding solderregions on the PCB. Thereafter, with the non-chip-bearing surface of themodule facing the PCB, the solder regions on the module are brought intocontact with the corresponding contact pads or solder regions on the PCBand the resulting assembly is heated in order to melt, and therebyreflow, the solder regions on the chip and/or on the PCB. In general,the magnitude of the difference between the CTE of the module and theCTE of the PCB is relatively small, and therefore the resulting solderconnections between the module and the PCB need not be encapsulated inan encapsulant.

If the module of interest has electrically conductive pins extendingfrom the non-chip-bearing surface of the module, then the module ismounted onto a PCB by, for example, initially positioning the moduleover the top (i.e., the circuit-bearing) surface of the printed circuitboard and inserting the electrically conductive pins of the module intocorresponding, copper plated through holes (PTHs) extending through thethickness of the PCB. Then, the PCB and the module are placed on aconveyor, which passes the PCB and module over a fluxing wave or fluxsprayer, which serves to impinge liquid flux onto the bottom surface ofthe PCB and into the PTHs. This flux is wicked up into the PTHs, andthus flux is applied to both the walls of the PTHs and to the pinsextending into the PTHs. Thereafter, the conveyor passes the PCB andmodule over a solder wave, which serves to impinge liquid solder ontothe bottom surface of the of the PCB and into the PTHs. This liquidsolder is also wicked up into the PTHs, filling the PTHs and, uponcooling and solidification, serving to encapsulate the pins within thePTHs.

One of the most important aspects of the above-described chip-mountingand module-mounting procedures is the choice of flux. That is, as notedabove, the flux serves to remove any oxide layers which may have formedon the solder regions, contact pads, pins or PTHs and to increase thewetting of, for example, contact pads by solder regions. In mostinstances, at the completion of the soldering process, use of thecommonly available fluxes results in ionic residues remaining on thesolder regions, contact pads, pins or PTHs. Such ionic residues areundesirable because they lead to corrosion of circuitry and to shortcircuits. Consequently, if formed, such ionic residues must be removed,e.g., cleaned with water, after the completion of the soldering process.

The solder connections formed between a chip and a PCB or between apinless module and a PCB, as described above, have relatively smallheights, e.g., 4 mils, and therefore the spacing between a chip orpinless module and its PCB is correspondingly small. This is significantbecause it implies that it would be very difficult, if not impossible,to clean away any ionic residues remaining on the solder regions and/orcontact pads after the completion of the soldering process. In addition,in the case of a pinned module, while corresponding ionic residues arereadily cleaned with water, one must then deal with the environmentalhazards posed by the resulting contaminated water.

Significantly, those engaged in the development of fluxes and solderingprocesses for mounting chips and modules onto PCBs have sought, thus farwith little success, fluxes which leave essentially no ionic residues onsolder regions, contact pads, pins or PTHs at the completion of thecorresponding soldering processes.

SUMMARY OF THE INVENTION

The invention involves a new flux composition which leaves essentiallyno ionic residues at the completion of conventional soldering processesused to mount electronic components, such as chips and chip carriermodules, onto PCBs. Significantly, this composition includes pimelicacid (HOOC(CH₂)₅ COOH) as the primary active ingredient, i.e., as theprimary fluxing agent. In addition, this composition includes twoorganic solvents. The first organic solvent has a relatively lowevaporation temperature, e.g., 82.4 degrees C., and is preferablyisopropanol (isopropyl alcohol). The second organic solvent has arelatively high evaporation temperature, e.g., about 170 degrees C., andis preferably propylene glycol monobutyl ether (also denominatedN-butylpropylglycol ether). While not essential, the compositionpreferably also includes a relatively small amount of water, preferablyde-ionized water.

The invention also involves the application of the new flux compositionto soldering processes used to mount electronic components, such aschips, chip carrier modules, resistors, capacitors, etc., onto PCBs.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the sole, accompanyingfigure, not drawn to scale, denominated FIG. 1 for convenience, whichdepicts the steps involved in mounting a semiconductor chip, in theso-called flip-chip configuration, onto a PCB, in accordance with theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention involves a new flux composition which leaves essentiallyno ionic residues at the completion of conventional soldering processesused to mount electronic components onto PCBs. Consequently, there is noneed to clean away such ionic residues at the completion of thesesoldering processes with, for example, water, and therefore there is noneed to deal with the environmental hazards posed by water contaminatedwith ionic residues.

The invention also involves the application of the new flux compositionto soldering processes used to mount electronic components, such aschips, chip carrier modules, resistors, capacitors, etc. onto PCBs.

Significantly, the new flux composition includes pimelic acid(HOOC(CH₂)₅ COOH) as the primary active ingredient, i.e., as the primaryfluxing agent. (It should be noted that at room temperature, pimelicacid is a solid, and that pimelic acid has a melting temperature ofabout 105 degrees C.) In addition, this new flux composition includestwo organic solvents, the first of which has a relatively lowevaporation temperature, e.g., 82.4 degrees C., and the second of whichhas a relatively high evaporation temperature, e.g., about 170 degreesC. While not essential, the new flux composition preferably alsoincludes a relatively small amount of water, preferably de-ionizedwater. It should be noted that the pimelic acid, the second organicsolvent and the water are soluble in the first organic solvent.

The first organic solvent is preferably isopropanol (isopropyl alcohol),which has an evaporation temperature of 82.4 degrees C. Usefulalternatives to isopropanol include n-propanol and benzyl alcohol. Asnoted above, the pimelic acid, the second organic solvent, describedbelow, and the water, if present, are soluble in the first organicsolvent.

The second organic solvent is preferably propylene glycol monobutylether (also denominated N-butylpropylglycol ether), which has anevaporation temperature of about 170 degrees C. Useful alternatives topropylene glycol monobutyl ether include propylene glycol monopropylether and diethylene glycol monomethyl ether. It should be noted thatupon evaporation of the first organic solvent during the solderingprocesses described below, the pimelic acid (and the water, if present)is then substantially dissolved in the second organic solvent, until thesecond organic solvent evaporates during the soldering processes.

The relative amount of pimelic acid in the inventive flux compositionranges from about 1% to about 6% by weight. If the inventive fluxcomposition is to be used, for example, in soldering a semiconductorchip to a PCB, then the relative amount of pimelic acid is preferably4.5% by weight. Relative amounts of pimelic acid less than about 1% byweight are undesirable because they result in insufficient and/orinadequate fluxing action, i.e., insufficient and/or inadequate removalof oxide layers and insufficient reductions in solder surface tensions.Relative amounts of pimelic acid greater than about 6% by weight areundesirable because they result in undesirably large amounts of residuesat the completion of conventional soldering processes.

The relative amount of the first organic solvent, e.g., isopropanol, inthe inventive flux composition ranges from about 25% to about 75% byweight. If the inventive flux composition is to be used, for example, insoldering a semiconductor chip to a PCB, then the relative amount of thefirst organic solvent is preferably 70.9% by weight. Relative amounts ofthe first organic solvent less than about 25% by weight are undesirablebecause the corresponding flux compositions may not be single phasesystems and therefore the application of these compositions may resultin undesirable, non-uniform distributions of pimelic acid. Relativeamounts of the first organic solvent greater than about 75% by weightare undesirable because the corresponding concentrations of pimelic acidare undesirably low and therefore the corresponding fluxing actions areinadequate.

The relative amount of the second organic solvent, e.g., propyleneglycol monobutyl ether, in the inventive flux composition ranges fromabout 10% to about 35% by weight. If the inventive flux composition isto be used, for example, in soldering a semiconductor chip to a PCB,then the relative amount of the second organic solvent is preferably23.6% by weight. Relative amounts of the second organic solvent lessthan about 10% by weight are undesirable because the pimelic acid tendsto precipitate out of solution, resulting in undesirable, non-uniformdistributions of pimelic acid. Relative amounts of the second organicsolvent greater than about 35% by weight are undesirable because thecorresponding concentrations of pimelic acid are undesirably low and thecorresponding fluxing actions are inadequate.

The relative amount of water, if used, in the inventive flux compositionranges from 0% to about 2% by weight. The purpose of the water, ifpresent, is to provide positively charged ions to accelerate theinitiation of fluxing action by the pimelic acid. If the inventive fluxcomposition is to be used, for example, in soldering a semiconductorchip to a PCB, then the relative amount of water is preferably 0.9% byweight. Relative amounts of water greater than about 2% by weight areundesirable because this significantly increases the possibility thatthe application of the inventive flux composition will result in ionicresidues.

By way of example, one embodiment of the inventive flux compositionwhich is useful in soldering a semiconductor chip to a PCB is readilyformed by dissolving 4.8 grams of pimelic acid, 25.0 grams of propyleneglycol monobutyl ether and 1.0 grams of de-ionized water in 75.0 gramsof isopropanol.

As noted above, the invention involves not only the new fluxcomposition, per se, but also the application of the new fluxcomposition to the soldering processes used to mount electroniccomponents onto PCBs. Thus, for example, in mounting a semiconductorchip 10 (see FIG. 1(a)), in the so-called flip-chip configuration, ontoa PCB 50 (see FIG. 1(b)), contact pads 30 on the circuitry-bearingsurface 20 of the chip 10 are provided with solder regions, e.g., solderballs, 40. These solder regions 40 have compositions which include, forexample, 90 atomic percent lead (Pb) and 10 atomic percent tin (Sn).Significantly, such solder regions have melting temperatures of 281degrees C., and do not melt during the soldering process describedbelow.

As shown in FIG. 1(b), prior to soldering the chip 10 to the PCB 50,contact pads 70 on the circuitry-bearing surface 60 of the PCB 50 areprovided with relatively small solder regions, e.g., relatively smallsolder balls, 80. These relatively small solder regions 80 are readilytransported to, and deposited on, the contact pads 70 via a decal 90. Bycontrast with the solder regions 40 used with the chip 10, the solderregions 80 have compositions which include, for example, 37 atomicpercent Pb and 63 atomic percent Sn. These solder regions 80 havemelting temperatures of 183 degrees C. and do melt during the solderingprocess, described below.

Prior to soldering the chip 10 to the PCB 50, the inventive fluxcomposition is applied to the solder regions 80, and/or the solderregions 40, and/or the contact pads 70 on the PCB 50, and/or the contactpads 30 on the chip 10. This is readily accomplished using, for example,a syringe or a brush.

Having applied the inventive flux composition to the relevant solderregions and/or contact pads, and as depicted in FIG. 1(c), the chip 10is positioned relative to the PCB 50 so that the solder regions 40contact the solder regions 80. Consequently, these combined solderregions substantially extend from the chip contact pads 30 to the PCBcontact pads 70.

With the solder regions 40 and 80 touching each other, the chip 10/PCB50 assembly is heated in, for example, an oven. During this heatingprocedure, the oven temperature is initially raised to about 183 degreesC., and subsequently raised to about 250 degrees C. Then, the oventemperature is lowered to about 183 degrees C., and thereafter loweredto room temperature. As a consequence, the solder regions 80 undergomelting and flow around the solder regions 40, resulting in continuousmetallurgical and electrical connections between the PCB 50 and the chip10. While the cleaning of these continuous connections would beextremely difficult, and perhaps even impossible, no such cleaning isneeded because essentially no ionic residues remain at the completion ofthis soldering process.

At the completion of the above-described soldering process, thecontinuous solder connections between the PCB 50 and the chip 10 arepreferably encapsulated in, for example, an epoxy resin, usingconventional techniques.

If the electronic component to be mounted onto a PCB is, for example, a(pinless) chip carrier module bearing at least one semiconductor chip,then such a module is readily mounted by, for example, screening solderregions onto contact pads on the non-chip-bearing surface of the module.Such solder regions may also be screened onto corresponding contact padson the PCB. The inventive flux composition is then applied to the solderregions and/or the module contact pads and/or the PCB contact padsusing, for example, a syringe or a brush. Thereafter, the module ispositioned in relation to the PCB so that the solder regions on themodule contact pads touch the solder regions on the PCB contact pads.Thus, these combined solder regions substantially extend from the modulecontact pads to the PCB contact pads. Then, with the module solderregions touching the PCB solder regions, the module/PCB assembly isheated in, for example, an oven in order to melt the module solderregions and/or the PCB solder regions.

By contrast with the above, if the electronic component to be mountedonto a PCB is, for example, a pinned chip carrier module bearing atleast one semiconductor chip, then such a module is readily mounted byinitially applying the inventive flux composition to the module pinsand/or to the walls of corresponding PTHs in the PCB. This is readilyaccomplished (using any of a variety of conventional techniques) beforethe module pins are inserted into the PTHs, while the module pins arebeing inserted into the PTHs, or after the module pins are inserted intothe PTHs. Preferably, this is accomplished after the module pins havebeen inserted into the PTHs by, for example, placing the module/PCBassembly on a conveyor, which passes this assembly over a fluxing waveor a flux sprayer. This fluxing wave or flux sprayer serves to impingethe inventive flux composition onto the bottom surface of the PCB andinto the PTHs. The impinged flux is wicked up into the PTHs, and thusthe inventive flux composition is applied to both the walls of the PTHsand to the module pins. Thereafter, the conveyor preferably serves topass the module/PCB assembly over a solder wave, which serves to impingeliquid solder onto the bottom surface of the PCB and into the PTHs. Thisliquid solder is also wicked up into the PTHs, filling the PTHs and,upon cooling and solidification, serving to encapsulate the pins withinthe PTHs.

If the electronic component to be mounted onto a PCB is, for example, adiscrete, passive electronic component, such as an electrical resistoror capacitor, having leads instead of pins, then such an electroniccomponent is readily mounted using a procedure which is almost the sameas that used with a pinned chip carrier module. The only difference isthat the leads of the discrete, passive electronic component are notpositioned inside the PTHs. Rather, these leads are positioned adjacentthe PTHs, e.g., these leads are placed in contact with the landsencircling the PTHs. Thus, when the component/PCB assembly is passedover the fluxing wave or flux sprayer, the inventive flux composition iswicked up into the PTHs, onto the lands and onto the bottom portions ofthe leads. Similarly, when the component/PCB assembly is passed over thesolder wave, liquid solder is wicked up into the PTHs, onto the landsencircling the PTHs and onto the bottom portions of the leads.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:
 1. A single phase flux composition comprising:(a) 1 to 6% byweight pimelic acid, (b) 25 to 75% by weight of a first organic solvent,and (c) 10 to 35% by weight of a second organic solvent having a higherevaporation temperature than said first organic solvent, said secondorganic solvent being (i) soluble in said first organic solvent (ii)capable of substantially dissolving the pimelic acid in said compositionupon evaporation of said first organic solvent therefrom, and (iii)capable of dissolving any water in said composition in an amount of upto 2% by weight upon evaporation of said first organic solventtherefrom.
 2. The composition of claim 1, wherein said flux compositioncontains water in an amount of up to 2% by weight.
 3. The composition ofclaim 2, wherein said second solvent is selected from the groupconsisting of propylene glycol monobutyl ether, propylene glycolmonopropyl ether and diethylene glycol monomethyl ether.
 4. Thecomposition of claim 3, wherein said first organic solvent is selectedfrom the group consisting of isopropanol, n-propanol and benzyl alcohol.5. The composition of claim 1, wherein said second solvent is selectedfrom the group consisting of propylene glycol monobutyl ether, propyleneglycol monopropyl ether and diethylene glycol monomethyl ether.
 6. Thecomposition of claim 5, wherein said first organic solvent is selectedfrom the group consisting of isopropanol, n-propanol and benzyl alcohol.